Method for manufacturing printed wiring board

ABSTRACT

There is provided a method for manufacturing a flat printed wiring board in which spaces between circuit patterns are filled with a resin. The method comprises: laminating via a mold release film a plurality of sets of laminated bodies formed by superposing a semi-cured resin sheet on a printed wiring board with circuit patterns formed thereon; placing the laminated plural sets of the laminated bodies interposed between a pair of smoothing plates and collectively pressing the laminated bodies in a reduced pressure atmosphere used for curing the resin; and then polishing the cured resin covering the circuit patterns, thereby exposing the circuit patterns.

TECHNICAL FIELD

The present invention relates to a method for manufacturing a flatprinted wiring board in which spaces between circuit patterns are filledwith a resin.

BACKGROUND ART

In the case of manufacturing a multilayer printed wiring board by e.g. abuild-up method, it is necessary to flatten the surface of a lower layersubstrate in order to increase the wiring density. However, the circuitpatterns of a printed wiring board may be manufactured by a subtractivemethod for removing unnecessary parts of copper foil via etching. As aresult, the printed wiring boards are generally formed into an unevenconfiguration. The part representing the circuit pattern typically risesabove the surrounding surface of the base material.

Therefore, in order to flatten a printed board in which the surface isformed in an uneven form as described above, a following method has beenproposed. For example, a method comprising: laminating a semi-curedresin sheet onto circuit patterns; pressing the resin sheet in a reducedpressure atmosphere so as to have the resin fill in the spaces betweenthe circuit patterns, and curing the resin; and polishing the curedresin covering the circuit patterns, in order to obtain a smoothedsubstrate with exposed circuit patterns.

Heretofore, a laminating press has generally been utilized formanufacturing a multilayer substrate by pressing a laminated body. Thelaminated body is formed by laminating a prepreg and a metallic foil,etc., onto an uneven core substrate on which circuit patterns areformed. In the laminating press, which is intended for making amultilayered substrate (i.e. for laminating an insulation layer onto thecircuit patterns), there has been a condition in which the pressing mustbe performed while adjusting the pressing pressure and the pressingamount (i.e., distance), etc., in order to make the insulating layer onthe circuit patterns have a predetermined thickness. In the abovedescribed circumstances, the pressing condition may be so demanding thatit is difficult to collectively press a plurality of sets of laminatedbodies. More specifically, in the case where a plurality of sets oflaminated bodies are arranged to be collectively pressed in order toform a number of individual laminated bodies with a predeterminedthickness, various conditions such as the pressing pressure and the heattransfer rate in each laminated body need to be made uniform across thewhole group of laminated bodies. To this end, smoothing plates must bearranged between respective laminated bodies. Accordingly, aconfiguration for performing collective pressing via the arranging ofsmoothing plates between respective laminated bodies has been generallyadopted in the art of laminated pressing.

However, the above described method of performing collective pressing byarranging smoothing plates between respective laminated bodies has aproblem in that the number of laminated bodies that can be received by apressing machine with a fixed width is reduced by the thickness of theplurality of smoothing plates. As a result, the productivity level ofthe laminating press is lowered.

The present invention has been made in view of the above describedcircumstances. An object of the present invention is to provide a methodfor manufacturing a flat printed wiring board and with a relativelyexcellent level of productivity.

DISCLOSURE OF THE INVENTION

In order to solve the above described problem, according to the presentinvention there is provided a method for manufacturing a flat printedwiring board in which the spaces between circuit patterns are filledwith a resin. The method comprises: laminating a plurality of sets oflaminated bodies, in which the sets of laminated bodies are formed bysuperposing a semi-cured resin sheet onto a printed wiring board withcircuit patterns formed thereon, via a mold release film; placing thelaminated plural sets of laminated bodies so as to be interposed betweena pair of smoothing plates and then pressing the laminated plural setsof laminated bodies in a reduced pressure atmosphere in order to curethe resin; and then polishing the cured resin covering the circuitpatterns, thereby exposing the circuit patterns.

The circuit patterns may be formed on both sides of the printed wiringboard. A metallic foil with a roughened surface facing the resin layermay also be superposed on the resin layer. In this case, the metallicfoil may be formed from a different type of metal than the type of metalof the circuit pattern.

The object of the present invention is not to make a multilayeredsubstrate by forming an insulating layer on circuit patterns as in theconventional case, but to provide a flat substrate with exposed circuitpatterns by filling the spaces between the circuit patterns with resin.Accordingly, the adjustment of the thickness of the resin layer on thecircuit patterns is not required. The resin only needs to be pressed tothe very limit of the height of the circuit patterns so as to leave aslittle resin as possible on the circuit patterns and to fill the resininto the spaces between the circuit patterns. As a result, a pair ofsmoothing plates only needs to be arranged on the outermost parts of theplurality of sets of laminated bodies. In this way, by reducing thenumber of smoothing plates at the time of pressing it is possible toincrease the number of laminated bodies received in a pressing machinehaving a predetermined width. It is also possible to improve the thermalcirculation at the time of heating. Consequently, the productivity canbe greatly improved. Further, a similar operating effect can also beobtained even when the above described circuit patterns are formed onboth sides of a substrate.

Additionally, a metallic foil with a roughened surface facing the resinmay be interposed between the smoothing plate and the resin prior to thetime of pressing the smoothing plate against the resin on the substrate.This allows the resin to more easily spread thinly and also causes thesurface of the resin to be formed in a fine uneven state correspondingto the roughened surface of the metallic foil. As a result, the residualresin layer can be more easily polished.

Still further, in the case where the metallic foil interposed betweenthe smoothing plate and the resin is formed with a different kind ofmetal from the kind of metal used in the circuit patterns, the metalfoil may be removed by selective etching. The selective etching may beused to only dissolve the metallic foil without affecting the metal ofthe circuit patterns.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a sectional view of a copper clad laminated board;

FIG. 2 is a sectional view of the wiring board with circuit patternsformed thereon;

FIG. 3 is a sectional view of a wiring board on which resin layers areformed with resin sheets according to an embodiment of the presentinvention;

FIG. 4 is a schematic view of a layout at the time of vacuum pressing;

FIG. 5 is a sectional view of the wiring board after the resin is cured;

FIG. 6 is a sectional view of the wiring board after the metallic foilsare removed; and

FIG. 7 is a sectional view of the wiring board after polishing.

BEST MODE FOR CARRYING OUT THE INVENTION

In the present embodiment, as shown in FIG. 1, a copper clad laminatedboard 10 is used as a base material in which copper foils 12 are adheredto both sides of a glass epoxy substrate 11 with a thickness of 100 μmto 3000 μm. Circuit patterns 15 are formed on the copper clad laminatedboard 10 by a known photoetching method (see FIG. 2).

Next, as shown in FIG. 3, resin layers 16 are formed on the substrate bylaminating resin sheets 20 with a thickness of about 30 μm, which areformed by e.g. semi-cured thermosetting epoxy resin, onto the circuitpatterns 15 of the wiring board. On the resin sheets 20, nickel foils17, having a thickness of 18 μm and one face of which is roughened by aneedle shaped plating, are also laminated beforehand such that theroughened surface faces the resin sheet 20. At this time, minute airbubbles may also be contained in the resin layer 16. Additionally, thesurface of the resin layers 16 is in a gradually rising/falling (i.e.,undulating) state as the surface of the resin layers 16 pass over theraised part of the circuit patterns 15. Thirteen sets of laminatedbodies 30 as described above are prepared and superposed via theintermediary of Tedlar 31 as a mold release film.

Next, as shown in FIG. 4, a pair of mirror surface plates 32 with athickness of about 1 mm are arranged via Tedlars 31 at the outermostparts of the thirteen sets of laminated bodies 30. On the mirror surfaceplates 32, kraft papers 33 (e.g., with a thickness of 0.25 mm×5 sheets)as a cushioning material, stainless jig plates 34 (e.g., with athickness of 4 or 5 mm) and top boards 35 (e.g., with a thickness of 4mm) as heat insulating materials, are respectively laminated in thissequence. The entire assembly of materials is placed on a carrier plate36 so as to be covered by a cap 37. After having been thusly arranged,the assembly of materials including the carrier plate 36 and the cap 37are then placed at a predetermined position in a pressing machine inorder to be pressed at a pressure of approximately 30 kg/cm² in areduced pressure atmosphere. As a result, the surface of the resinlayers 16, which is in a gradually rising/falling state, is compressed.The resin on the circuit patterns 15 is moved so as to fill the spacesbetween the circuit patterns. As a result, the substrate as a whole isflattened. The air bubbles in the resin layers 16 rise up to thevicinity of the surface of the resin layers 16 so as to be removed frominside of the resin.

After the resin layers 16 on the circuit patterns 15 are sufficientlycompressed and the air bubbles in the resin are fully released to theoutside, the resin layers 16 are heated so as to be finally cured.

When the resin is completely cured, the carrier plate 36 is carried outfrom the pressing machine and the laminated bodies 30 are removed. Thenickel foils 17 adhered to the surfaces of the resin layers 16 of eachlaminated body 30 are then removed by an etching solution usedexclusively for nickel (see FIGS. 5 and 6). As a result, the thicknessof the resin layers remaining on the copper circuit patterns 15 becomes10 μm or less and the surface of the resin is in a roughened state. Inorder to flatten the substrate, primary smooth surface polishing isperformed by ceramic buff polishing to remove the resin layers 16 fromthe circuit patterns 15. Secondary finish polishing is then performed bya surface grinding machine in order to bring the average roughnessaccuracy of the surface to become equal to or less than 3 μm (see FIG.7). In the case of surface polishing, since the resin layers 16remaining on the circuit patterns 15 have an extremely thin thickness(e.g., such as 10 μm) and the surface of the resin layers is roughened,the surface polishing is easily performed.

In this way, according to the present embodiment, a flat printed wiringboard with exposed circuit patterns can be manufactured with good levelof productivity.

The present invention is not to be limited to the embodiments describedabove with reference to the drawings. For example, the followingembodiments are also considered as included within the scope of thepresent invention. In addition, further variations other than thefollowing embodiments are possible within the scope and spirit of theinvention.

(1) In the above described embodiment, the circuit patterns are formedby the subtractive method. However, the circuit patterns may also beformed by an additive method.

(2) In the above described embodiment, a thermosetting epoxy resin isused as the material of the resin layers, but the embodiment is notlimited to this material. A thermosetting resin, such as a urea resin, amelamine resin, a phenol resin, an acrylic resin, and an unsaturatedpolyester resin may also be used.

(3) In the above described embodiment, nickel is used as the metallicfoil material, but the embodiment is not limited to this material. Othermetals such as copper may also be used.

INDUSTRIAL APPLICABILITY

As described above, according to the present invention a flat printedwiring board with exposed circuit patterns can be manufactured with agood level of productivity.

1. A method for manufacturing a flat printed wiring board in whichspaces between circuit patterns are filled with a resin, said methodcomprising the following steps: laminating via a mold release film aplurality of sets of laminated bodies formed by superposing a semi-curedresin sheet on a printed wiring board with said circuit patterns formedthereon; placing the laminated plural sets of said laminated bodiesinterposed between a pair of smoothing plates; collectively pressingsaid laminated bodies in a reduced pressure atmosphere; curing saidresin; and polishing said cured resin covering said circuit patterns,thereby exposing said circuit patterns.
 2. The method for manufacturingthe flat printed wiring board according to claim 1, wherein said circuitpatterns are formed on both sides of said printed wiring board.
 3. Themethod for manufacturing the flat printed wiring board according toclaim 2, including a further step of superposing a metallic foil with aroughened surface facing said resin layer onto said resin layer.
 4. Themethod for manufacturing the flat printed wiring board according toclaim 3, wherein said metallic foil is formed with a different kind ofmetal than the kind of metal used to form said circuit pattern.
 5. Themethod for manufacturing the flat printed wiring board according toclaim 4, wherein said metallic foil is nickel.
 6. The method formanufacturing the flat printed wiring board according to claim 3 whereinthe step of polishing comprises the steps of: ceramic buff polishing toremove resin layers from the circuit pattern; and finish polishing toreduce the average roughness of the surface of the flat printed wiringboard.
 7. The method for manufacturing the flat printed wiring boardaccording to claim 1 wherein said circuit patterns are formed by anadditive method.
 8. A method for manufacturing a flat printed wiringboard in which spaces between circuit patterns are filled with a resin,said method comprising the following steps: superposing a semi-curedresin sheet on said circuit pattern printed on a printed wiring board toform a board assembly; stacking a predetermined number of said boardassemblies with a mold release film interposed between every adjacentsaid board assembly to create a stack of board assemblies; superposing afirst smoothing plate on a first surface of said stack of boardassemblies via a first mold release film; superposing a second smoothingplate on a second surface of said stack of board assemblies via a secondmold release film; pressing said first and second smoothing plates andsaid stack of board assemblies in a reduced atmosphere environment;heating said stack of board assemblies so as to cure the semi-curedresin sheets; polishing each board assembly so as to expose eachcorresponding said circuit pattern.
 9. The method for manufacturing theflat printed wiring board according to claim 8, wherein said printedwiring board of said board assembly comprises a first board surface anda second board surface, and wherein said circuit pattern is printed onsaid first board surface, and wherein a second circuit pattern isprinted on said second board surface, and wherein a second semi-curedresin sheet is superposed on said second circuit pattern, and whereineach board assembly is further polished to expose each correspondingsecond circuit pattern.
 10. The method for manufacturing the flatprinted wiring board of claim 9 further comprising the steps of:superposing a metallic foil with a roughened surface onto saidsemi-cured resin sheet wherein the roughened surface contacts saidsemi-cured resin sheet, and superposing a second metallic foil with asecond roughened surface onto said second semi-cured resin sheet whereinthe second roughened surface contacts said second semi-cured resinsheet.
 11. A method for manufacturing a flat printed wiring boardcomprising the steps of: stacking a semi-cured resin sheet on each of asurface of a printed wiring board comprising a printed circuit pattern;stacking a metallic foil with a roughened surface onto each of thesemi-cured resin sheets wherein the roughened surface contacts thecorresponding semi-cured resin sheet; stacking a predetermined number ofthe printed wiring boards comprising said corresponding semi-cured resinsheets, said corresponding metallic foils, and said correspondingcircuit patterns, wherein adjacent printed wiring boards are separatedby a corresponding mold release film and additional said correspondingmold release films are also placed over a first end and a second end ofsaid stack of printed wiring boards; placing said stack of printedwiring boards and said corresponding mold release films between a firstsmoothing plate and a second smoothing plate; compressing said firstsmoothing plate and said second smoothing plate and said correspondingstack of printed wiring boards and said corresponding mold release filmsin a reduced pressure environment so as to reduce a thickness of each ofa combination of printed wiring board and corresponding semi-cured resinsheets; heating said stack of printed wiring boards so as to cure eachsaid corresponding semi-cured resin sheets; removing said correspondingmetallic foils from each printed wiring board; polishing each printedwiring board to expose said corresponding circuit patterns.
 12. Themethod for manufacturing the flat printed wiring board of claim 11,wherein said printed circuit pattern comprises: a first printed circuitpattern printed on a first surface of the printed wiring board; and asecond printed circuit pattern printed on a second surface of theprinted wiring board.
 13. The method for manufacturing the flat printedwiring board of claim 12, wherein said first printed circuit pattern andsaid second printed circuit pattern are respectively formed by asubtractive method.
 14. The method for manufacturing the flat printedwiring board of claim 12, wherein a thermosetting epoxy resin is used asa material of said semi-cured resin sheets.
 15. The method formanufacturing the flat printed wiring board of claim 12, wherein athermosetting resin is used as a material of said semi-cured resinsheets.
 16. The method for manufacturing the flat printed wiring boardaccording to claim 1, including a further step of superposing a metallicfoil with a roughened surface facing said resin layer onto said resinlayer.
 17. The method for manufacturing the flat printed wiring boardaccording to claim 16, wherein said metallic foil is formed with adifferent kind of metal than the kind of metal used to form said circuitpattern.